Fuel injector control systems and methods for internal combustion engines are well-known in the art, for instance from U.S. Pat. Nos. 4,459,670, 4,355,619 and EP-A-0 114 378. Conventional solenoid fuel injectors are provided with solenoid actuators and comprise a valve housing with current coil and electrical connections, a valve seat with a nozzle and a movable valve needle. When such an injector is energized (e.g., a current is sent to the solenoid actuator), the coil generates a magnetic field which lifts the valve needle off of its seat to allow fuel to flow through the injector and to leak out of the nozzle towards the combustion chamber of the associated cylinder. When the injector is de-energized (e.g., the current is no longer sent to the solenoid actuator), the valve needle is pressed against the valve seat.
In order to cause a fuel injection to occur, a nominal start time for the injection and a nominal energizing time (ET) for the injector are predetermined by an electronic injection control unit, taking into account several parameters, such as for instance the amount of fuel to be injected, the engine speed, the engine power, the exhaust emissions. Referring to the graphs of FIG. 1, a voltage V having a predetermined nominal start value Vpull-in (“pull-in voltage”) is applied to the injector solenoid at the nominal energizing time (te) to cause in said solenoid the flow of an energizing current I which very quickly increases from zero to a predetermined peak value (Ipull-in or “pull-in current”) in a period of time, which is defined as the “pull-in period”. The quick increase of the energizing current allows the coil to generate a sufficiently strong force to move a mechanical anchor within the injector, in order to permit the starting movement of the injector needle.
In order to achieve a very fast movement of the anchor the solenoid current I must reach the peak value Ipull-in as quickly as possible. For this reason it is common practice to apply to the injector an initial voltage having a value Vpull-in which is much higher (for instance, between 50 V and 70 V) than the typical voltage value VBATT (for instance 12 V) available from the battery of the motor-vehicle. The higher initial voltage value may be obtained with per se known booster circuits.
Still referring to the graphs of FIG. 1, at the end of the pull-in period, when the anchor lift is opened, there is usually no longer any need to maintain the current I at the peak value, since the fuel pressure itself is now capable to provide part of the force which is necessary to maintain the anchor opened. For this reason, the injector current can be reduced, as shown in the upper graph of FIG. 1, wherein the current I in the injector solenoid is reduced gradually, to subsequent lower levels.
The gradient of the solenoid current I in the pull-in period is essentially proportional to the value Vpull-in of the voltage applied to the injector. That value Vpull-in of the voltage V is generally set through the hardware, but for some reasons in the operation it can be lower than expected, because of the variations of some parameters such as the environmental temperature, the aging of hardware components, etc. If the actual value of the pull-in voltage Vpull-in is lower than the expected nominal value, the slew rate or gradient of the current I in the pull-in period is correspondingly reduced, and the corresponding duration of the pull-in period is correspondingly increased, as shown by three exemplary graphs presented in FIG. 2. As a consequence, the anchor lift movement is correspondingly delayed, as shown by the three exemplary graphs showing the displacement L of the anchor in FIG. 2 (wherein Lc indicates the position of the anchor when the injector is closed and LO indicates the position of the anchor when the injector is open).
Thus, when the pull-in voltage is lower than the expected nominal value, the actual injection starts with a certain delay with respect to the nominal energizing time and the fuel quantity which is actually injected is different from, and in particular smaller than, the expected quantity.
At the end of the energizing time set by the injection control unit, the current I in the injector solenoid must be brought to zero as fast as possible, in order to reduce any further delays and the corresponding differences between the quantity of fuel actually injected and the expected quantity.
In order to bring the injector current I to zero as fast as possible, a voltage having the same magnitude of the initial pull-in voltage is conveniently applied to the injector, however with the opposite or reversed polarity.
Another problem which may arise with the methods according to the prior art is represented by the time distance between two subsequent injections. In some cases, because of the low speed of the anchor in the closing displacement, when two subsequent injections are very close to one another, the two injection times can merge together, producing a total amount of fuel injected which is far from the expected quantity.
In view of the above, it is at least one object of the present invention to improve the accuracy with which a solenoid-actuated fuel injector is driven. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the